The present invention relates to a golf ball having a core in which a gas generated by thermal decomposition of an organic peroxide creates a foamed region, and to a method of manufacture thereof.
Rubber or resin compositions based on synthetic rubbers such as polybutadiene or on various types of thermoplastic resins have hitherto been used as golf ball materials. A number of golf balls have been disclosed in which, for the purpose of improving distance performance and feel at impact, some constituent portion of the ball is in a foamed form.
For example, U.S. Pat. No. 6,688,991 discloses a golf ball having a core which contains a highly neutralized resin material that is foamed in order to control the moment of inertia of the ball. However, this art involves foaming a resin; given that resins generally have a lower resilience than rubbers and that foaming such a resin material lowers the resilience even further, a drawback of such golf balls is that the distance traveled by the ball is greatly reduced.
Also, JP No. 3958833 discloses art wherein a two-layer core has a center core that is produced from a rubber composition containing a blowing agent. Yet, in this art, foaming cannot be carried out only in a target range within a single layer of the core. In addition, the two-layer construction of the core increases the production costs.
U.S. Pat. No. 5,688,192 discloses a golf ball having a compressible gaseous material dispersed at the interior. Also, JP No. 5166056 discloses art that includes, within a core-forming rubber composition, thermally expandable microcapsules containing a large amount of gas. However, in these disclosures, the compressible gaseous material and the thermally expandable microcapsules sometimes collapse due to the pressure applied during rubber curing, or may not properly expand, presenting difficulties during manufacture.